Field of the Invention
The present invention relates to spin forming process and to apparatus for manufacturing articles by spin forming. The invention has particular, but not necessarily exclusive, application to metal spinning.
Related Art
Metal spinning refers to a group of forming processes that allow production of hollow, axially symmetric (axisymmetric) sheet metal components. The basic technique of spinning, which is common to this group of processes, consists of clamping a sheet metal blank against a mandrel on a spinning lathe, and gradually forming the blank onto the mandrel surface by a roller, either in a single step or series of steps.
A detailed review of academic literature related to spin forming has been carried out and disclosed by Music et at (2010) [O. Music, J. M. Allwood, K. Kawai “A review of the mechanics of metal spinning” Journal of Materials Processing Technology 210 (2010) 3-23], the entire content of which is hereby incorporated by reference.
It is of interest here to draw a distinction between the terms conventional spinning, shear spinning and tube spinning, all of which are considered to be spin forming processes. A common feature of the three processes is that they typically allow production of hollow, rotationally symmetric parts. The main difference between the three is apparent in the wall thickness of the formed part. In conventional spinning, the wall thickness remains nearly constant throughout the process, so the final wall thickness of the formed part is substantially equal to the thickness of the blank. In contrast, the wall thickness is reduced in shear spinning and tube spinning; in shear spinning, part thickness is dictated by the angle between the wall of the component and the axis of rotation; in tube spinning, the final thickness is defined by the increase in length of the workpiece. Furthermore, while in conventional spinning and tube spinning parts can be formed in a single step or a number of steps, in shear spinning, forming is done in a single step.
A conventional spinning process is illustrated in FIG. 1, in which initial sheet metal workpiece 10 is held in a metal spinning apparatus clamped between a tailstock 12 and a mandrel 14. The mandrel 14, sheet metal workpiece 10 and tailstock are rotatable about principal rotational axis A. The spinning sheet is pressed towards the mandrel 14 using roller 16, supported by roller arm 18 and rotatable about roller axis X. FIG. 2 shows examples of feasible geometries formable by known conventional metal spinning processes. All are axisymmetric, and as can be seen, the range of feasible axisymmetric shapes is relatively broad.
A shear spinning process is illustrated in FIG. 3. Initial sheet metal workpiece 20 has thickness t0. Initial sheet metal workpiece 20 is held in a metal spinning apparatus clamped between a tailstock 22 and a mandrel 24. The mandrel 24, sheet metal workpiece 20 and tailstock are rotatable about principal rotational axis A. The spinning sheet 20a is pressed towards the mandrel 24 using roller 26, supported by roller arm 28. In the shear spinning process, the thickness of the metal workpiece is reduced substantially, to t1, where t1<t0. In some shear spinning processes, the overall diameter of the workpiece (measured perpendicular to the axis A) is the same after the spinning process as before the spinning process. The limit of shear spinning is given by the minimum angle α that can be achieved in the finished geometry, where:
As α is decreased, the required reduction in wall thickness to achieve the required value for α becomes very significant, leading to failure of the workpiece where the required value for α is too low. FIG. 4 shows examples of feasible geometries formable by shear spinning processes.
Some workers have recognised that metal spinning is limited to the production of axisymmetric geometries. Therefore some work has been done to try to modify metal spinning processes in order to produce non-axisymmetric geometries.
For example. US 2005/0183484 discloses the use of a control system in order to control the pressing force of a roller tool against a workpiece where the mandrel has a non-axisymmetric geometry. During the process, the workpiece conforms to the outer shape of the mandrel. A similar process is set out in US 2008/0022741.